“Crews entered a bedroom [on the third floor] and saw the ceiling was glowing orange,” ­Bastis said.

A good Samaritan in one of the complex’s five neighboring buildings reportedly noticed the roof of the first building on fire and called one of its residents just after 9 p.m., residents said. That person then alerted the rest of the building’s residents by knocking on doors.

Samantha Fox, 25, was putting her two children to bed in her second-­floor apartment when she heard a knock on the door from her neighbor Ruby, who said the roof was on fire and everyone needed to get out.

“There were no fire alarms, no smoke detectors, no warning,” Fox said.

The cause of the fire was ­unclear Monday.

Matthew Tansey Jr. said his ­only alert was the knocks on his basement-floor apartment’s windows and door from neighbors.

“If it wasn’t for that, we wouldn’t have heard anything,” said Tansey, 22. “There was absolutely no mechanical warning.”

Once outside, Fox said the building’s residents watched as flames crawled from the roof to the basement. The building’s roof eventually caved in. Fox credits Ruby and the person from the neighboring building who first ­noticed the fire with preventing a catastrophe.

Much of the fire initially burned walls behind the apartments or the ceiling cocklofts, none of which are equipped with alarms. The building’s main alarm system is connected to smoke ­detectors in the common halls, and with smoke mainly confined to the walls, there was nothing to detect,
Bastis said.

“It allowed plenty of time for people to get out, once we realized we actually had a fire,” ­Bastis said.

Bastis said the situation could have been a lot worse.

None of the building’s residents were injured. One firefighter suffered a fall and another ­required stitches for a hand laceration.

The Red Cross provided temporary shelter to 11 children and seven adult residents of the 24-unit building, said spokeswoman Ashley E. ­Studley.

Volunteers also provided funds for clothing and food to a total of 30 residents left homeless by the Hart Street fire, she said.

The three-story building has a wood frame and was built in 1974, city assessor records show.

Bastis estimated that the fire caused between $250,000 and $500,000 in damage and said that with the combination of smoke damage, water damage, a caved roof, and major electrical problems, the building is uninhabitable.